Issue 38

H. Weil et alii, Frattura ed Integrità Strutturale, 38 (2016) 61-66; DOI: 10.3221/IGF-ESIS.38.08 62 The validation of the model is performed on rotating fatigue bending specimens. Indeed specimens with dedicated geometry are explored to introduce stress concentration at the material surface aiming a greater probability of rupture in the nitrided layer, and thus fatigue testing of the nitrided layer rather than the core. The work is also conducted on gears designed by Hispano-Suiza, a company of the SAFRAN Group. M ULTIAXIAL FATIGUE CRITERIA he fatigue criteria E allow taking into account the mechanical characteristics of the nitriding layers. During this study, the choice has been made to select the linear criterion in the following form [6, 7]: * * app H σ αP E β   (1) The applied stress app *  is calculated with respect to the loading the material undergoes. The residual stresses and the increased hardness induced by nitriding are taken into account in the hysdrostatic pressure H P * and coefficient  [8, 9]. The slope  of the straight line is calculated using fatigue properties of the non-nitrided material. Hypothetically, the coefficient  does not depend on the gradient of mechanical properties provided by nitriding [5]. Two Wöhler curves with different load ratio can then be used to characterize the fatigue properties for a given failure probability. The prediction of failure follows from Eq.1. If E < 1 , no failure will take place [10]. Otherwise, crack initiation and failure might occur. The Crossland [6] and Dang Van [11-13] criteria were used in order to predict the fatigue life of the nitride materials. Crossland criterion The Crossland criterion (Eq. 2) is the most used and has been studied in the case of nitrided materials (Eq. 2) [6]. 2a c Hmax c c J α P E β   (2) It uses the maximum hydrostatic pressure Hmax P and the second invariant of the alternating (alternative) stress a J 2 . Because of the relation between a J 2 and alt eqVM  (Eq. 3), the Crossland criterion is considered as a von Mises criterion. alt eq VM 2a σ J 3  (3) Dang Van criterion In case of a calculation algorithm without the determination of critical plane of the material, the second version of the Dang Van criterion is used (Eq. 4) [12, 14].     DV H DV t T DV τ t α P t E max β          (4) where t is the time of load cycle. The shear stress component   t  is equal to the Tresca stress. Thus, it is calculated with the eigenvalues of the stress tensor (Eq. 5).                 1 2 2 3 3 1 1 τ t max σ t σ t , σ t σ t , σ t σ t  2     (5) T